(1) Field of the Invention
This invention relates to random access decoders, and in particular to a random access decoder for use in an imaging array readout circuit.
(2) Description of the Art
In many devices, such as visible or infra-red imaging detectors, it is necessary to read information from a two dimensional array of detector pixels. This can be achieved using a series of switches to transport data from particular pixel of the two dimensional array on to a common bus line. The two dimensional array of pixel elements used in infra-red imaging arrays are usually incorporated with row and column readout circuitry on a single silicon readout chip. Each pixel element may also comprise additional electronic components such as amplifiers, noise filters etc. Such chips are typically fabricated using complementary metal oxide semiconductor (CMOS) technology.
One type of row and/or column readout circuitry commonly used in imaging detectors is a shift register. A shift register typically comprises a chain of flip-flop type shift register elements. A logic “1” is loaded into the first shift register element, and each shift register element is then sequentially enabled by a series of clock pulses.
A two dimensional array incorporating row and column shift registers is read by extracting data from the first pixel of the first row through to the last pixel in the first row and then repeating the process for each subsequent row. In this way, the entire array is sequentially read by rastering through each pixel in turn. As shift registers only operate sequentially, they are unable to randomly access pixels in a two dimensional array. The inability to randomly read data from any pixel in the array is a disadvantage, especially in larger arrays, when data from only a particular portion or “window” of the array is of interest.
Another type of row and/or column readout circuitry is a decoder. Decoders typically comprise a plurality of unique decoder circuit elements, and any of these decoder elements can be accessed as required. Decoders, unlike shift registers, will thus allow truly random access to any pixel in a two dimensional array.
A disadvantage of known decoders is the requirement to design a plurality of unique decoder elements; such design work is time consuming and may also be complex when using non-binary array sizes. The requirement to produce a custom design for each decoder element also adds to the cost and complexity of fabricating the readout circuitry. For example, the requirement for unique row and column decoder circuit elements limits the maximum array size that can be produced using known CMOS fabrication techniques.
For a more complete review of the multiplexing techniques used in infrared detector applications see chapter 5 of the Infrared and electro-optical handbook, vol 3, Electro-optical components, W. D. Rogatto, SPIE Optical Engineering Press, Bellingham, Wash.